1. Field of the Invention
This invention relates to a servomotor control device for controlling the frequency of a servomotor according to information on the actual frequency of the servomotor.
2. Description of the Background Art
To control servomotor in a DAT (digital audio tape recorder), VCR (video cassette recorder), etc., for example, a servomotor for driving a rotating head drum, a servomotor for driving a capstand and a servomotor for driving a tape reel and the like use a signal indicative of the actual frequency of the servomotor. The signal indicative of frequency of a servomotor is a pulse signal which is generated once or more during one revolution of the servomotor by sensing, with a sensor (frequency generator), one or more magnetic poles mounted on a rotating mechanism. From an interval of such pulse signal, a frequency is calculated. This measured frequency is compared with a target frequency, and the difference between them is used to generate a control signal for the servomotor.
For such control signal, in the case of an AC motor, an analog waveform signal produced by a PWM (pulse width modulation) circuit, for example, is used, and in the case of a DC motor an analog voltage signal produced by a PWM circuit or a D-A (digital to analog) converter or the like, is used.
A conventional servomotor control device as described above, in which a frequency is calculated from an interval of a pulse signal received from a frequency generator and compared a with a target frequency to generate a servomotor control signal, has a disadvantage described below.
Because of a time delay between the moment when the pulse signal is supplied from the frequency generator and the moment when the servomotor control signal is calculated, which occurs whether the above-mentioned calculation is executed by hardware or software, it is impossible to output, at the moment when the pulse signal is received, a control signal adapted for that moment. Generally, the time delay can be made a small value if servo control is implemented by hardware. In servo control by software, because the arithmetic operations include time-consuming operations such as multiplication in addition to addition and subtraction, the time delay becomes a considerable value.
Therefore, though the output control signal is the optimum value for the moment when the pulse signal is obtained, it differs from the optimum value for the moment when the control signal is outputted. As a result, problems such as a lowering of response speed of a servo control system and ringing occur such that the motor speed does not converge and its amplitude oscillates in progression, making it difficult for the servo system to stay in a stable condition.